A large majority of light detection applications today rely on low cost, lightweight, high performance integrated circuit devices, such as, CCD's (charge coupled devices), p-i-n (p-type semiconductor:insulator:n-type semiconductor) and avalanche photodiodes. However for applications which require detection of very small signals with low signal to noise ratios (SNR), the vacuum photomultiplier tube is still superior to these integrated circuit type photodetectors.
A schematic of a conventional photomultiplier is shown in FIG. 1. It consists of a photocathode (C) and a series of electrodes called "dynodes" 1-8. Each dynode is biased at a progressively higher voltage than the cathode. Typically, the voltage increase at each dynode is about 100 volts.
Photons striking the photocathode generate electrons via the photoelectric effect. These electrons are accelerated by the field between electrodes and strike the surface of the first dynode with an energy equal to the accelerating voltage. Each primary electron generates several secondary electrons in the collision with the surface of the first dynode. These secondary electrons are accelerated towards the second dynode and the process is repeated. After passing through about eight stages of dynodes, the single photoelectron will have grown to a packet of 10.sup.5 or 10.sup.6 electrons. The last electrode, labeled A, is the anode which collects the electrons in the final stage. The anode signal is then fed into appropriate external signal processing electronics. Two types of photocathodes that have been used are the opaque photocathode and the semitransparent photocathode which only partly absorb incident light and are schematically depicted in FIGS. 2 and 3, respectively. The spectral sensitivity of the photocathode is determined by its work function, therefore it is possible to choose a photocathode material to match a specific application.
Some of the disadvantages of conventional photomultipliers relative to integrated photodetectors are their large size and weight, high costs, and large power consumption. Furthermore, external electronics are normally required to obtain useful signal information. This requires additional interconnections, which increases system complexity and reliability. As a consequence, some modern applications, e.g. remote sensing, have been prohibited.
Thus, there is a continuing need in the state of the art for a microelectronic form of a photomultiplier tube which is designed to combine the desirable features of conventional photomultiplier tubes with the lightweight, low-power, low-cost advantages of an integrated circuit device.